1. Field of the Invention
This invention is generally related to the field of marine seismic operations and more particularly to the testing of marine seismic cables or streamers under actual prospecting conditions.
2. Description of the Prior Art
Marine seismic streamers are frequently a mile or more in length and are towed through the water behind an instrument-carrying vessel adapted to record seismic information transmitted along the cable from a series of pressure detectors disposed at intervals within such cable. The cable usually consists of many sections, each containing a group of transducers interconnected to provide a common output. If the cable is composed of a hundred such sections, there will be a hundred pairs of leads of gradually increasing length.
If any of the pressure detectors are defective or if their electrical connections with the recording equipment is damaged, the response of such detectors to seismic reflections may be either misleading or lost completely. It is therefore desirable to in some manner determine whether all the detectors in a marine streamer are in fact functional before embarking on an offshore seismic operation.
A streamer is normally carried on a reel on the deck of the two vessel to the location of interest. If the cable is electrically connected to the recording instruments, it is possible to perform a gross polarity check of each detector by pounding on the reel with a hammer. However, this is in no sense an adequate check of the quality or performance of individual detectors. A better technique is to submerge the reeled streamer beside the two vessel and activate an acoustic energy source mounted on the vessel. This will provide an indication of polarity and amplitude of the electrical output of each transducer. It is however subject to the disadvantage that it does not duplicate actual prospecting conditions. When the streamer is deployed behind the tow vessel, it may stretch and cause breaks, electrical discontinuities or other damage affecting the performance of the detectors.
What is needed therefore is an effective means and method for testing a marine streamer under actual working conditions immediately prior to a seismic run.
It should be noted that with computer techniques now available the interpretation of seismic records has been refined to a high degree. Therefore, anomalies in transducer performance which formerly were not significant may now seriously interfere with geophysical work.